Carbides

Carbides are binary compounds of carbon with elements of lower or about equal electronegativity. Carbides are classified into three types on the basis of chemical bonding. These are salt−like carbides, covalent carbide and interstitial or alloy like carbides. Carbides are generally prepared by heating the element or its oxide with carbon or hydrocarbon at very high temperatures.

Ba + 2C → BaC₂

2Li + 2C → Li₂C₂

CaO + 3C → CaC₂ + CO

4Li + C₂H₂ → Li2C₂ + 2LiH

Salt−like carbides

These carbides are formed by the metals of groups IA, IIA and IIIA (except boron), coinage metals, zinc, cadmium and some lanthanides. Depending on the nature of the hydrocarbon formed during hydrolysis by water or dilute HCl, the ionic carbides can be further classified into the following types.

(i) Acetylides: These are the ionic carbides which yield acetylene on hydrolysis. These are regarded as derivatives of acetylene and are thus called acetylides. The alkali metals and copper, silver and gold form M₂C₂ type compounds. The alkaline earth metals, zinc and cadmium give MC₂ type carbides. These carbides are believed to contain [C ≡ C]₂− groups. Two types of crystal structures characterize these carbides. Both are tetragonal and are derived from sodium chloride structure by replacement of Cl− ions by  C₂^2- ions.

(ii) Methanides: These carbides evolve methane on hydrolysis. These are regarded as derivatives of methane and are called methanides Al₄C₃, Be₂C, Mn₃C, etc. are some examples of methanides. These contain C4− groups.

(iii) Allylides: These carbides evolve allylene (methyl acetylene) on hydrolysis. The only example of this type is Mg₂C₃. It contains  C₃^4-discrete groups.

(iv) Mixed carbides: These carbides yield a mixture of hydrocarbons on hydrolysis. Carbides of iron group, UC₂ and ThC₂ belong to this class.